SCRs are commonly used to control large currents. When an SCR fires, and the SCR maintains a conducting state between its anode and cathode until the through current falls below a minimum holding current level. The gate to cathode voltage determines the anode to cathode voltage at which the SCR will fire. A normal method of operating an SCR device is to apply a positive and negative going AC voltage across the anode to cathode terminals, thus when the positive anode to cathode voltage is large enough, the SCR fires and when the anode to cathode voltage becomes zero, the gate regains control over the device. Prior art circuits either short circuit the gate to cathode terminals or apply a small positive voltage equal to the saturation voltage of a transistor across the gate to cathode terminals to hold the SCR in an off condition. These prior art techniques do not insure that the SCR will stay in a non-conducting state because with any small positive gate to cathode voltage, the SCR will still fire at a large positive anode to cathode voltage, or when the anode voltage rise per unit time is large. Thus prior circuits must prevent large anode to cathode voltages from being applied to the SCR. Even with zero volts applied between the gate and cathode terminals, the SCR may still fire at some positive anode to cathode voltage, and this anode to cathode voltage is extremely dependent upon temperature. Thus prior art circuits do not immunize an SCR firing control circuit against temperature variations.
Alternator regulator systems have used SCR control circuits to energize the field coil of the generator. Both the battery voltage and the output voltage of a generator winding have been used in prior art circuits to energize the field coil winding. In all cases where an SCR was used to control the field coil winding. In all cases where an SCR was used to control the field coil excitation, complex feedback circuits were necessary to insure that the SCR would turn off and stay in an off condition when the battery attained a fully charged condition.